Simplify the following expression: $ n = \dfrac{1}{9} + \dfrac{7}{r + 4} $
In order to add expressions, they must have a common denominator. Multiply the first expression by $\dfrac{r + 4}{r + 4}$ $ \dfrac{1}{9} \times \dfrac{r + 4}{r + 4} = \dfrac{r + 4}{9r + 36} $ Multiply the second expression by $\dfrac{9}{9}$ $ \dfrac{7}{r + 4} \times \dfrac{9}{9} = \dfrac{63}{9r + 36} $ Therefore $ n = \dfrac{r + 4}{9r + 36} + \dfrac{63}{9r + 36} $ Now the expressions have the same denominator we can simply add the numerators: $n = \dfrac{r + 4 + 63}{9r + 36} $ $n = \dfrac{r + 67}{9r + 36}$